Do Lava Rocks Lower Nitrate?

[Siggy G]

I used to keep koi and aquarium fish and I remember one of the advantages if porous biofilter media was that in the interior nitrate could be converted to nitrogen and there by lower it's concentration. I filled two 55 gal. drums with lava rock for a bio filter for my koi pond for that exact reason and it seemed to work well. But I wonder if anyone has taken this into consideration for aquaponics, seeing that nitrate is not a problem but a necessity for plant growth. My initial thoughts are that denitrification doesn't reduce enough nitrate to be a problem. But has anyone given this any thought?

[David]

When there is an anaerobic spot in your media one of the things that establishes itself is denitrifying bacteria.  These always exist anyway and people fail to mention them when talking about bugs that make this whole process work.  These denitrifying bacteria release nitrogen back into the atmosphere as a gas and help balance the nitrifying bactera which release nitrogen into the media.  The overgrowth of the former usually will deplete fertility in the grow bed and its presence will be pronounced by that good ole fashioned "stank" which ain't good.  As it takes an anaerobic area for these bacteria to grow it's obvious to conclude that this area would not be inhabited much by the good bugs.

In a well built system both nitrifying and denitrifying bacteria will be in balance.

Could this possibly be what you meant?

[Siggy G]

Yes, that's what I was refering to. I'm just curious if using porous media in your filter or your beds causes nitrate deficiencies? I can imagine for example that a system set up with porous media in their growing beds would contain a lot of these bacteria and could be sapping a substantial amount of nitrate out of the system.

David said:

When there is an anaerobic spot in your media one of the things that establishes itself is denitrifying bacteria.  These always exist anyway and people fail to mention them when talking about bugs that make this whole process work.  These denitrifying bacteria release nitrogen back into the atmosphere as a gas and help balance the nitrifying bactera which release nitrogen into the media.  The overgrowth of the former usually will deplete fertility in the grow bed and its presence will be pronounced by that good ole fashioned "stank" which ain't good.  As it takes an anaerobic area for these bacteria to grow it's obvious to conclude that this area would not be inhabited much by the good bugs.

In a well built system both nitrifying and denitrifying bacteria will be in balance.

Could this possibly be what you meant?

[David]

Quite the contrary.  The more surface area the greater amount of beneficial bacteria which produces a greater biofilter for the AP system.  Dr Lennard Wilson has a spreadsheet calculator on his website.  If you download this document you can easily see the surface area measurements for porous vs regular rock or other organic media.

Now I am not saying lava rock is the best.  I just happen to like it.  It's a combination of weight issues, surface area and aesthetics which is why I use it.  Other people have used granite as well as pea gravel and get fantastic results all the same.

And for the record I have to keep pruning plants back because they grow out of control in my AP system.  I wouldn't exactly classify that as a nutrient deficiency or lack of nitrogen. :0)

[Siggy G]

So it would seem that the denitrification doesn't have a negative effect on aquaponics.
Thanks for the info. I'm just playing with ideas and thinking the whole thing through before I actually build anything.

David said:

Quite the contrary.  The more surface area the greater amount of beneficial bacteria which produces a greater biofilter for the AP system.  Dr Lennard Wilson has a spreadsheet calculator on his website.  If you download this document you can easily see the surface area measurements for porous vs regular rock or other organic media.

Now I am not saying lava rock is the best.  I just happen to like it.  It's a combination of weight issues, surface area and aesthetics which is why I use it.  Other people have used granite as well as pea gravel and get fantastic results all the same.

And for the record I have to keep pruning plants back because they grow out of control in my AP system.  I wouldn't exactly classify that as a nutrient deficiency or lack of nitrogen. :0)

[David]

Correct. It doesnt seem to effect it in a negative manner unless thewater dissolved oxygen becomes too low. At that point it will go anaerobic and start stinking. You will definitely know something is a foot because it will smell worse than gym socks.